Critical zone architecture predisposes nitrate routing while rainfall controls export magnitude in a karst hillslope

In karst hillslopes, the factors controlling where nitrate is routed may differ from those controlling how much nitrate is exported, but this functional separation remains poorly resolved. We monitored surface runoff (SR), subsurface runoff at the soil–epikarst interface (SsR), and epikarst runoff at the epikarst–bedrock interface (EpR) on a karst hillslope platform in southwest China. Across 10 plots and 47 erosive rainfall events, we quantified runoff partitioning and dissolved N export, with water and nitrate isotopes measured during 18 and three events, respectively. EpR had the highest runoff coefficient (0.18 ± 0.15), whereas SR had the highest dissolved N concentrations. EpR dominated annualized NO 3 − -N export (0.47 ± 0.34 kg N ha −1 yr −1 ) because its greater water flux outweighed its lower nitrate concentration. Water-isotope compositions differed systematically among pathways, consistent with pathway-dependent storage and mixing, while nitrate-isotope patterns pointed to overlapping source mixing and subsurface processing. Hierarchical partitioning identified critical-zone structure as the largest contributor to NO 3 − -N partitioning to EpR (39.3% of explained variance), with meteorological factors contributing a comparable share (35.6%), whereas meteorology clearly dominated total NO 3 − -N export (63.9%). With increasing rainfall magnitude, nitrate export shifted from EpR dominance toward greater contributions from SR and SsR, while land-use contrasts were most pronounced under storm rainfall. Together, these results reveal a functional separation in which critical-zone architecture predisposes nitrate routing, whereas rainfall-driven hydrological connectivity governs export magnitude. Accurate assessment of nitrate loss and groundwater contamination risk in karst hillslopes therefore requires multi-interface monitoring across the critical zone.